1. Field of the Invention
The present invention relates to a waveform shaping apparatus that shapes a waveform of a signal beam, and more specifically, to a waveform shaping apparatus, an optical transmission system, and a waveform shaping method that can simultaneously shape waveforms of a plurality of signal beams each having a wavelength assigned to a channel.
2. Description of the Related Art
To prevent waveform degradation of a signal beam transmitted over an optical fiber, various waveform shaping technologies have been thought out and commercialized.
One example of the waveform shaping technologies is disclosed in Japanese Patent Application Laid-open No. 2003-15097. FIG. 24 is a schematic for explaining a waveform shaping technology according to this disclosure. This document discloses a waveform shaping technology that shapes a waveform of a signal beam using two optical gates (OGs) (a first OG and a second OG).
As shown in FIG. 24, a signal beam is input to the first OG together with a probe beam emitted from a laser diode (LD). By driving a bulk electroabsorption modulator by a reverse bias as the first OG, data in the signal beam is transferred to the probe by cross-absorption modulation (XAM) effect.
Due to screening or band-filling effects of a photoexcited carrier, saturation occurs in absorption, and the data in the signal beam is transferred to the probe non-linearly with respect to the power of the signal beam, thus suppressing a space level noise resulted from wavelength dispersion. Because the beam transferred with the data has a different wavelength from that of the original signal beam, the former can be separated from the latter at an optical bandpass filter (BPF). By sending the signal beam separated at the BPF to the second OG (a gain clamp amplifier such as a distributed feedback (DFB) laser), a mark level noise resulted from dispersion can be suppressed (waveform shaping can be provided).
However, due to the underlining principle, the above-described waveform shaping technology cannot provide simultaneous waveform shaping for WDM signal beams.
In other words, it has become important to achieve a waveform shaping technology that can shape waveforms of a plurality of signal beams each having a wavelength assigned to a channel.